A polymer electrolyte fuel cell has a structure composed of a polymer electrolyte membrane, a catalyst layer on each side of the electrolyte membrane, and a gas diffusion layer on the outer side of each catalyst layer. The catalyst layer is generally a porous layer formed of a particulate support having a noble metal catalyst supported on the surface thereof. The porous layer allows a fuel gas, such as hydrogen or methanol, or an oxidizing gas, such as oxygen or air, to pass therethrough to cause an electrode reaction on the three-phase interface, forming water in the catalyst layer.
While the water formed is to dissipate from the catalyst layer, it sometimes accumulates in the catalyst layer. If water accumulation proceeds to exceed the tolerance of the catalyst layer, what we call “flooding” occurs. Conversely, if the catalyst layer has too much drainage, it would dry out. With the object of providing a fuel cell electrode catalyst layer that satisfactorily meets the conflicting demands for good drainage of generated water and moderate moisture retention, Patent Literature 1 below proposes a porous electrode catalyst layer having first pores with a diameter of 0.01 to 0.1 μm and second pores with a diameter of 0.1 to 1 μm. The electrode catalyst layer disclosed contains carbon as a catalyst support.
With a view to providing an electrode catalyst layer that exhibits enhanced water retention without adversely affecting reaction gas diffusion and drainage of water generated by the electrode reaction and thereby achieves high power generation characteristics even under a low humidity condition, Patent Literature 2 below proposes an electrode catalyst layer which comprises a polymer electrolyte and particles having a catalyst substance supported thereon and in which the volume of pores of 1.0 μm or smaller in diameter as measured by mercury intrusion porosimetry and converted by cylindrical approximation increases continuously in the thickness direction from the outside (the surface of the electrode catalyst layer) toward the inside (the polymer electrolyte membrane). The catalyst support contained in the catalyst layer of Patent Literature 2 is carbon similarly to Patent Literature 1.